Self-assembly of a short amphiphile in water controlled by superchaotropic polyoxometalates: H4SiW12O40 vs. H3PW12O40

Abstract

Nanometric ions, such as polyoxometalates (POMs) or ionic boron clusters, with low charge density have previously shown a strong propensity to bind to macrocycles and to adsorb to neutral surfaces: micellar, surfactant covered water-air and polymer surfaces. These association phenomena were shown to arise from a solvent-mediated effect called the (super-)chaotropic effect. We show here by combining cloud point (CP) measurements, scattering (SAXS/SANS) and spectroscopic techniques (NMR) that Keggin POMs: H₄SiW₁₂O₄₀ (SiW) and H₃PW₁₂O₄₀ (PW), induce the self-assembly of an organic solvent: dipropylene glycol n-propylether (C₃P₂), in water. The strong interaction between SiW/PW with C₃P₂ leads to a drastic increase in the CP, and aqueous solubility, of C₃P₂, e.g. SiW enables reaching full water-C₃P₂ co-miscibility at room temperature. At high POM concentrations, SiW leads to a continuous increase of the CP, forming SiW-[C₃P₂]_1-2 complexes, whereas PW produces a decrease in the CP attributed to the formation of nearly "dry" spherical [PW]_n[C₃P₂]_m colloids, with n ~ 4 and m ~ 30. At high C₃P₂/PW contents, the [PW]_n[C₃P₂]_m colloids turn into large interconnected structures, delimiting two pseudo-phases: a PW-C₃P₂-rich phase and a water-rich phase. It is proposed that the stronger electrostatic repulsions between SiW (4-), compared to PW (3-), prevents the formation of mesoscopic colloids.

Keywords: Adsorption; Colloids; Mesoscopic structuring; Polyoxometalates; Self-assembly; Superchaotropicity.